This invention relates to the preparation of ceramic materials or articles by the pyrolysis of preceramic polysilanes wherein the preceramic polysilanes are rendered infusible prior to pyrolysis by exposure to ultraviolet irradiation in the presence of a reactive gas selected from the group consisting of ethylene, 1,3-butadiene, 2-methyl-1,3-butadiene, 1,4-pentadiene, silane, chlorosilane, dichlorosilane, boron trichloride, oxygen and water mixture, acetylene, and tetravinylsilane. This method is especially suited for the preparation of ceramic fibers. The process of this invention can produce ceramic fibers with modified surface properties.
Ceramic materials have been prepared by the pyrolysis of various preceramic polymers in the prior art. Baney et al. in U.S. Pat. No. 4,310,651 (issued Jan. 12, 1982) disclosed a polysilane of general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and chlorine atoms or bromine atoms. The polysilane was converted to a beta-silicon carbide containing ceramic at elevated temperatures (about 1400.degree. C.) without any separate step to render the polysilane infusible prior to pyrolysis.
Baney et al. in U.S. Pat. No. 4,298,559 (issued Nov. 3, 1981) prepared polysilanes of general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and additional alkyl radicals of 1 to 4 carbon atoms or phenyl radicals. Upon heating these polysilanes were converted into silicon carbide containing ceramics in high yields. No method was disclosed whereby the polysilane could be rendered infusible prior to pyrolysis.
Baney et al. in U.S. Pat. No. Re. 31,447 (reissued Nov. 22, 1983) disclosed polysilanes of the general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and alkoxy radicals containing 1 to 4 carbon atoms or phenoxy radicals. Silicon carbide ceramics were obtained by firing these polysilanes to elevated temperatures. Again, no method for rendering the polysilane infusible was disclosed.
Baney et al. in U.S. Pat. No. 4,314,956 (issued Feb. 9, 1982) disclosed polysilanes of the general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to silicon and amine radicals of the general formula --NHR.sup.iv where R.sup.iv is a hydrogen atom, an alkyl radical of 1 to 4 carbon atoms or a phenyl radical. A silicon carbide ceramic was obtained by firing this polysilane to an elevated temperature under an inert atmosphere without a separate step to render the polysilane infusible.
The polysilanes of U.S. Pat. Nos. 4,310,651, 4,298,599, Re 31,447, and 4,314,956 are further discussed in Baney et al. Organometallics, 2, 859 (1983).
West in U.S. Pat. No. 4,260,780 (issued Apr. 7, 1981) prepared a polysilane of general formula EQU [(CH.sub.3).sub.2 Si][CH.sub.3 (C.sub.6 H.sub.5)Si]
by the sodium metal reduction of dimethyldichlorosilane and methylphenyldichlorosilane. The resulting polysilanes had very high softening points (&gt;280.degree. C.). No infusibilty step was disclosed.
West et al. in Polym. Prepr., 25, 4 (1984) disclosed the preparation of a polysilane of general formula EQU [CH.sub.3 (CH.sub.2 .dbd.CHCH.sub.2)Si][CH.sub.3 (C.sub.6 H.sub.5)Si]
by the sodium metal reduction of allylmethyldichlorosilane and methylphenyldichlorosilane. These polysilanes were rapidly gelled with ultraviolet irradiation.
Seyferth et al. in U.S. Pat. No. 4,639,501 (issued Jan. 27, 1987) prepared preceramic polymers by reacting a methylpolysilane of the general formula [(RSiH).sub.x (RSi).sub.y ].sub.n with an organosilicon compound having at least two vinyl groups of the general formula [R.sub.2 (CH.sub.2 .dbd.CH)Si].sub.2 Y, where, for example, Y is O, S, NH, NR, or is absent, using either UV irradiation, thermal energy, or catalysts. No procedure was given for rendering the preceramic polymer infusible prior to pyrolysis.
Gaul in U.S. Pat. No. 4,312,970 (issued Jan. 26, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting organochlorosilanes and disilazanes. The preceramic silazane polymers were pyrolyzed in an inert atmosphere without any separate treatment to render the silazane preceramic polymer infusible.
Gaul in U.S. Pat. No. 4,340,619 (issued July 20, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting chlorine-containing disilanes and disilazanes. Fibers prepared from such preceramic silazane polymers were given a "mild heat treatment" in air before pyrolysis but there is no teaching that such a treatment rendered the fibers infusible.
Cannady in U.S. Pat. No. 4,540,803 (issued Sept. 10, 1985) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting trichlorosilane and disilazane. The preceramic silazane polymers were not rendered infusible prior to pyrolysis to form ceramic materials.
Bartos et al. in U.S. patent application Ser. No. 748,109, filed June 24, 1985, and now abandoned obtained infusible preceramic silazane polymers by treatment of the preceramic silazane polymer with a steam or a steam and oxygen mixture.
Lu in a copending U.S. patent application Ser. No. 69,565, filed July 6, 1987, and now U.S. Pat. No. 4,847,027, entitled "Infusible Preceramic Polymers via Nitric Oxide Treatment" describes the preparation of ceramic materials or articles by the pyrolysis of preceramic polymers wherein the preceramic polymers are rendered infusible prior to pyrolysis by exposure to gaseous nitric oxide. The preceramic polymers include polycarbosilanes, polysilazanes, and hydridopolysilazanes.
Lipowitz, in U.S. patent application Ser. No. 926,168, filed Nov. 3, 1986, now U.S. Pat. No. 4,743,662 obtained infusible preceramic polymer by treatment of the preceramic polymer with a plasma energy source. Both vinyl-containing and vinyl-free preceramic polymers were rendered infusible by plasma treatment. High energy electrons interact with neutral gas molecules in the plasma thereby forming unique species such as metastable species, atoms, radicals, and ions. Theses unique species then interact with the preceramic polymer thereby rendering the preceramic polymer infusible.
Lutz et al., in U.S. patent application Ser. No. 905,020, filed Sept. 8, 1986, now U.S. Pat. No. 4,816,497, obtained infusible preceramic polymer compositions by treatment of the compositions with UV irradiation wherein the compositions contained vinyl- or allyl-containing preceramic polymers, mercapto compounds, and photoinitiators.
Bujalski et al., in copending U.S. patent applications Ser. No. 94,347, filed Sept. 8, 1987, and now U.S. Pat. No. 4,889,899, entitled "A Method of Producing Silicon Carbide Preceramic Vinyl-containing Polymers" and Ser. No. 94,434, filed Sept. 4, 1987, now U.S. Pat. No. Des. 321,091, and entitled "An Improved Method of Producing Silicon Carbide Preceramic Vinyl-containing Polymers," obtained infusible preceramic polymers by thermal treatment of vinyl-containing polysilanes or by UV irradiation of vinyl-containing polysilanes.
Yajima et al. in U.S. Pat. Nos. 4,220,600 (issued Sept. 2, 1980), 4,283,376 (issued Aug. 11, 1981), 4,342,712 (issued Aug. 3, 1982), and 4,399,232 (issued Aug. 16, 1983) disclosed the use of gamma ray or electron beam irradiation to cure preceramic modified polycarbosilane fibers prior to pyrolysis.
What has been newly discovered is a method of rendering preceramic polysilanes infusible prior to pyrolysis by ultraviolet irradiation of the preceramic polysilanes in the presence of a reactive gas. This method represents a significant advance in the art of preparing ceramic materials or articles, especially in the art of preparing ceramic fibers.